The invention relates to an arrangement for storing an execution history in an information processing unit which includes execution addresses for a microprogram.
Generally, a conventional information processing unit using a microprogram is provided with an arrangement in which a tracer memory stores the history of execution addresses for the microprogram and specific logical signals as clues to the identification of the cause of any exceptional event that may occur in the unit. In such an information processing unit, a microinstruction read out of a control memory is stored in a microinstruction register, and decoded by a decoder, which produces control signals for the execution of this microinstruction. Whereas these operations are successively repeated to form a sequence of the microprogram, if an exceptional event is detected, the storage operation of the tracer memory is suspended, and the inquiry into the cause of the exceptional event occurring in the unit is facilitated by analyzing the execution history of the microprogram stored in the tracer memory. Since the tracer memory is limited in memory capacity, it cannot store all the execution history starting when the unit is activated. For this reason, if the whole storage area of the tracer memory is occupied with history data, the subsequent new history data are written over the old ones.
A recent information processing unit provided with the above-described arrangement in which a higher reliability is required does not suspend its operation even if an exceptional event occurs, but performs a failure recovery procedure such as an instruction retry. Such a unit has the disadvantage that, if a second exceptional event arises while a recovery procedure for the first exceptional event is in progress and if another recovery procedure for the second exceptional event is in turn started, the tracer memory may become fully occupied by the execution history for the second exceptional event, leaving no execution history for the first exceptional event and making it difficult to identify its cause.
An object of the invention is, therefore, to provide an arrangement for storing an execution history in an information processing unit free from the above-mentioned disadvantage of the conventional arrangement.